10. Wireless Networks

Transcription

1 Computernetzwerke und Sicherheit (CS221) 10. Wireless Networks 1. April 2011 omas Meyer Departement Mathematik und Informatik, Universität Basel Chapter 6 Wireless and Mobile Networks (with changes CS221 UniBasel, 2011) A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). ey re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. ey obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. anks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach, International Version, 5 th edition. Jim Kurose, Keith Ross Pearson Addison-Wesley, March

2 Chapter 6: Wireless and Mobile Networks Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! # wireless Internet-connected devices soon to exceed # wireline Internet-connected devices laptops, Internet-enabled phones promise anytime untethered Internet access two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes point of attachment to network 3 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( Wi-Fi ) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9 Summary 4

3 Elements of a wireless network network infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (nonmobile) or mobile wireless does not always mean mobility 5 Elements of a wireless network network infrastructure base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers, access points 6

4 Elements of a wireless network network infrastructure wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance 7 Characteristics of selected wireless link standards n Data rate (Mbps) a,g b a,g point-to-point (WiMAX) UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO UMTS/WCDMA, CDMA2000 3G data 3G cellular enhanced.056 IS-95, CDMA, GSM 2G Indoor 10-30m Outdoor m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km 8

5 Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network 9 Elements of a wireless network ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves 10

6 Wireless network taxonomy infrastructure (e.g., APs) no infrastructure single hop host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets), PAN multiple hops host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET 11 Wireless Link Characteristics (1) Differences from wired link. decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times. make communication across (even a point to point) wireless link much more difficult 12

7 Wireless Link Characteristics (2) SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs given physical layer: increase power - > increase SNR->decrease BER given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) 13 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal strength C s signal strength Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B space Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B 14

8 Code Division Multiple Access (CDMA) used in several wireless broadcast channels (cellular, satellite, etc) standards unique code assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) 15 CDMA Encode/Decode channel output Z i,m sender data bits code d 1 = d 0 = slot 1 slot 0 Z i,m = d i. cm slot 1 channel output slot 0 channel output received input receiver code slot 1 slot 0 M D i = Σ Z. i,m cm m=1 M d 1 = -1 slot 1 channel output d 0 = 1 slot 0 channel output 16

9 CDMA: two-sender interference 17 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( Wi-Fi ) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9 Summary 18

10 IEEE Wireless LAN b GHz unlicensed spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps n: multiple antennae GHz range up to 200 Mbps all use CSMA/CA for multiple access all have base-station and ad-hoc network versions LAN architecture BSS 1 AP Internet AP hub, switch or router wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only BSS 2 20

11 802.11: Channels, association b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP s name (SSID) and MAC address selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in AP s subnet : passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 AP AP 2 AP AP 2 H1 H1 Passive Scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent: H1 to selected AP Active Scanning: (1)Probe Request frame broadcast from H1 (2)Probes response frame sent from APs (3)Association Request frame sent: H1 to selected AP (4)Association Response frame sent: H1 to selected AP 22

12 IEEE : multiple access avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C A B A s signal strength C s signal strength space 23 IEEE MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then sender receiver transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle data transmit when timer expires if no ACK, increase random backoff interval, repeat receiver ACK SIFS - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) 24

13 Avoiding collisions (more) idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! 25 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) RTS(B) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) 26

14 frame: addressing frame control duration address 1 address 2 address 3 seq control address 4 payload CRC Address 1: MAC address of wireless host or AP to receive this frame Address 2: MAC address of wireless host or AP transmitting this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode frame: addressing H1 R1 router Internet AP R1 MAC addr dest. address H1 MAC addr source address frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame 28

15 Not covered Advanced WiFi features (power management, WiMax,...) GSM, UMTS (and IP) Mobile IP 29